The treatment of heavy concentrations of nitrogen contained in a waste liquid effluent such as liquid pig manure (1 600 to 3 100 mg TKN/L) can give rise to substantial quantities of transformation byproducts, such as nitrates. When the treated liquid manure is released directly into the environment by means of a cleaning field, the authorities are increasingly scrutinizing the nitrogen concentrations in the effluents and more harsh standards are being examined, such as that reported by Dubé et al. in “Adaptation et Demonstration du procédé BIOSORMD—Lisier dans une ferme porcine du type finisseur”, Vecteur Environnement, 38(4), July (2005) 20-25. Thus, the work of transformation should be pushed to its limit to achieve very low nitrogen concentrations (e.g.: <20 mg N—NH4/L, <100 mg N—NO3/L) exiting the system.
With the technology of biofiltration on organic substrate, treatment of the liquid pig manure for a release into the environment is generally done by using two consecutive stages (primary biofilter and polishing biofilter). In this process, the mechanisms of nitrification (NH4→NO3) and denitrification (NO3→N2) take place simultaneously, as discussed by Garzón-Zúñiga et al. in “Nitrogen Elimination Mechanisms in an Aerated Peat Biofilter Treating Pig Manure”, Environ Technol, 26(4), (2005) 361-371, and by Gilbert et al. in “Nitrifying and Denitrifying Biomass Characterization of an Organic Biofilter Treating Pig Manure”, R. Stuetz and T. T. Lim (Editors), Water and Environmental Management Series—Young Researchers, (2006) 257-264. However, the very low release constraints for ammoniacal nitrogen (NH4) mean that the nitrification within the primary biofilter needs to be maximized, which can lead to quite elevated concentrations of nitrates at the outlet (on the order of 500 mg N—NO3/L). Consequently, one must promote the denitrification within the polishing biofilter while ensuring a complete transformation of the residual ammoniacal nitrogen. Denitrification by heterotrophic bacteria is accomplished in optimal manner under anoxic conditions and in presence of easily assimilable carbon, as discussed by Beline et al. in “Application of the 15N Technique to Determine the Contributions of Nitrification and Denitrification to the Flux of Nitrous Oxide from Aerated Pig Slurry”, Water Research, 35(11), (2001) 2774-2778. The low load of residual organic matter at the outlet of the primary biofilter and, thus, at the entrance of the polishing filter, appears to limit the denitrification, as mentioned by Aubry et al. in “Nitrogen Behavior in a Trickling Biofilter Treating Pig Manure”, Biofilm Systems Conference VI, 24-27 Sep. (2006), Aquatech, Amsterdam, Holland, and might account for the low transformations observed in the polishing unit.
The present invention thus proposes conditions of biofiltration adapted to simultaneous nitrification-denitrification in order to produce a substantially purified effluent containing a minimum of nitrogen in all its forms.